Democrats’ single most important domestic proposal – universal health insurance – may blow up in Barack Obama’s face when voters are exposed to the deadly details.

Obama has said, proudly and often, “I am going to give health insurance to 47 million Americans who are now without coverage.” But are they “Americans?”

That 47 million statistic includes illegal immigrants – who virtually all lack insurance. In fact, about one in four of those lacking insurance is here illegally. And they are, by far, the group most in need of health insurance.

About 15 million of the remaining uninsured are eligible for Medicaid but haven’t signed up – mainly because they haven’t gotten sick. When they do, they enroll in Medicaid and we pick up the full tab for their health care relatively cheaply. (About 80 percent of each Medicaid dollar goes to nursing-home care for the elderly, only about 20 percent for the medical needs of the poor.)

The rest of the uninsured pool? Virtually all the children are eligible for the State Children’s Health Insurance Program. Some aren’t enrolled because the parents haven’t bothered, but most are eligible. That leaves about 20 million uninsured adults who are US citizens or legal immigrants. There are far better ways to handle their needs than to turn our entire health-care system upside down.

Care for illegals is the biggest unmet medical need in our nation, and Obama’s program targets it squarely. But do we really want to give them federally paid coverage equal to what US senators get, as Obama proposes?

Covering illegals adds dramatically to the cost of any program – and would encourage more folks to enter America illicitly.

Obama’s plan will likely have a horrific effect on some local health-care systems.

Illegals now get free emergency-room treatment for life-threatening conditions – as any other American who’s entered an ER in an area with lots of illegals recently well knows. (Three-quarters of the illegal-immigrant population is concentrated in five states: California, New York, Florida, Texas and Illinois.)

But now they’d be eligible for the entire range of medical services, all free of charge. That would trigger severe rationing: bureaucrats deciding who gets to see an oncologist, who can have an MRI – and even who can have bypass surgery and who’d die for lack of it.

These decisions would be made not on the basis of legal status but on the brutal facts of triage: Treat the 37-year-old illegal with his whole life to live before you spend scarce resources on an overweight, diabetic, 80-year-old citizen with high blood pressure who smokes.

John McCain hasn’t raised this issue, perhaps for fear of offending the Latino vote. But polling suggests the case against rationing of health care would be as persuasive to Hispanic-American citizens as it is to the rest of us. Nobody wants to die waiting in line – especially not behind someone who snuck in ahead of us.